This study is a physicochemical study of protein-protein interactions in the E. coli ribosomal subunits. The proteins are isolated from the subunits and individually characterized for behavior free in solution. These proteins are mixed and observed for the presence of complex by means of analytical ultracentrifugation. The resultant quantitation of such mixtures leads to the determination of the free energy of association between proteins. It can be extrapolated that some of these interactions may actually occur within the integral structure of the subunits as well. Proteins currently being examined are S3, S4, S5, S7, S8, S9, S19 from the small subunit and L7/L17 and L10 from the large subunit. The study requires the detailed analysis of sedimentation equilibrium data on complex mixtures. Therefore, the study requires and involves the development of methods that facilitate the resolution of such data. The protiens L7/L12 and L10 form an unusual 4:1 complex which is apparently quite stable in urea solution up to 6 M. The factors that govern this stability will be examined and attempts at assessing the kinetics of the formation and disruption will be made employing conventional light-scattering methodology.